1 // SPDX-License-Identifier: GPL-2.0 2 /* 3 * SafeSetID Linux Security Module 4 * 5 * Author: Micah Morton <mortonm@chromium.org> 6 * 7 * Copyright (C) 2018 The Chromium OS Authors. 8 * 9 * This program is free software; you can redistribute it and/or modify 10 * it under the terms of the GNU General Public License version 2, as 11 * published by the Free Software Foundation. 12 * 13 */ 14 15 #define pr_fmt(fmt) "SafeSetID: " fmt 16 17 #include <linux/lsm_hooks.h> 18 #include <linux/module.h> 19 #include <linux/ptrace.h> 20 #include <linux/sched/task_stack.h> 21 #include <linux/security.h> 22 #include "lsm.h" 23 24 /* Flag indicating whether initialization completed */ 25 int safesetid_initialized __initdata; 26 27 struct setid_ruleset __rcu *safesetid_setuid_rules; 28 struct setid_ruleset __rcu *safesetid_setgid_rules; 29 30 31 /* Compute a decision for a transition from @src to @dst under @policy. */ 32 enum sid_policy_type _setid_policy_lookup(struct setid_ruleset *policy, 33 kid_t src, kid_t dst) 34 { 35 struct setid_rule *rule; 36 enum sid_policy_type result = SIDPOL_DEFAULT; 37 38 if (policy->type == UID) { 39 hash_for_each_possible(policy->rules, rule, next, __kuid_val(src.uid)) { 40 if (!uid_eq(rule->src_id.uid, src.uid)) 41 continue; 42 if (uid_eq(rule->dst_id.uid, dst.uid)) 43 return SIDPOL_ALLOWED; 44 result = SIDPOL_CONSTRAINED; 45 } 46 } else if (policy->type == GID) { 47 hash_for_each_possible(policy->rules, rule, next, __kgid_val(src.gid)) { 48 if (!gid_eq(rule->src_id.gid, src.gid)) 49 continue; 50 if (gid_eq(rule->dst_id.gid, dst.gid)){ 51 return SIDPOL_ALLOWED; 52 } 53 result = SIDPOL_CONSTRAINED; 54 } 55 } else { 56 /* Should not reach here, report the ID as contrainsted */ 57 result = SIDPOL_CONSTRAINED; 58 } 59 return result; 60 } 61 62 /* 63 * Compute a decision for a transition from @src to @dst under the active 64 * policy. 65 */ 66 static enum sid_policy_type setid_policy_lookup(kid_t src, kid_t dst, enum setid_type new_type) 67 { 68 enum sid_policy_type result = SIDPOL_DEFAULT; 69 struct setid_ruleset *pol; 70 71 rcu_read_lock(); 72 if (new_type == UID) 73 pol = rcu_dereference(safesetid_setuid_rules); 74 else if (new_type == GID) 75 pol = rcu_dereference(safesetid_setgid_rules); 76 else { /* Should not reach here */ 77 result = SIDPOL_CONSTRAINED; 78 rcu_read_unlock(); 79 return result; 80 } 81 82 if (pol) { 83 pol->type = new_type; 84 result = _setid_policy_lookup(pol, src, dst); 85 } 86 rcu_read_unlock(); 87 return result; 88 } 89 90 static int safesetid_security_capable(const struct cred *cred, 91 struct user_namespace *ns, 92 int cap, 93 unsigned int opts) 94 { 95 /* We're only interested in CAP_SETUID and CAP_SETGID. */ 96 if (cap != CAP_SETUID && cap != CAP_SETGID) 97 return 0; 98 99 /* 100 * If CAP_SET{U/G}ID is currently used for a setid or setgroups syscall, we 101 * want to let it go through here; the real security check happens later, in 102 * the task_fix_set{u/g}id or task_fix_setgroups hooks. 103 */ 104 if ((opts & CAP_OPT_INSETID) != 0) 105 return 0; 106 107 switch (cap) { 108 case CAP_SETUID: 109 /* 110 * If no policy applies to this task, allow the use of CAP_SETUID for 111 * other purposes. 112 */ 113 if (setid_policy_lookup((kid_t){.uid = cred->uid}, INVALID_ID, UID) == SIDPOL_DEFAULT) 114 return 0; 115 /* 116 * Reject use of CAP_SETUID for functionality other than calling 117 * set*uid() (e.g. setting up userns uid mappings). 118 */ 119 pr_warn("Operation requires CAP_SETUID, which is not available to UID %u for operations besides approved set*uid transitions\n", 120 __kuid_val(cred->uid)); 121 return -EPERM; 122 case CAP_SETGID: 123 /* 124 * If no policy applies to this task, allow the use of CAP_SETGID for 125 * other purposes. 126 */ 127 if (setid_policy_lookup((kid_t){.gid = cred->gid}, INVALID_ID, GID) == SIDPOL_DEFAULT) 128 return 0; 129 /* 130 * Reject use of CAP_SETUID for functionality other than calling 131 * set*gid() (e.g. setting up userns gid mappings). 132 */ 133 pr_warn("Operation requires CAP_SETGID, which is not available to GID %u for operations besides approved set*gid transitions\n", 134 __kgid_val(cred->gid)); 135 return -EPERM; 136 default: 137 /* Error, the only capabilities were checking for is CAP_SETUID/GID */ 138 return 0; 139 } 140 return 0; 141 } 142 143 /* 144 * Check whether a caller with old credentials @old is allowed to switch to 145 * credentials that contain @new_id. 146 */ 147 static bool id_permitted_for_cred(const struct cred *old, kid_t new_id, enum setid_type new_type) 148 { 149 bool permitted; 150 151 /* If our old creds already had this ID in it, it's fine. */ 152 if (new_type == UID) { 153 if (uid_eq(new_id.uid, old->uid) || uid_eq(new_id.uid, old->euid) || 154 uid_eq(new_id.uid, old->suid)) 155 return true; 156 } else if (new_type == GID){ 157 if (gid_eq(new_id.gid, old->gid) || gid_eq(new_id.gid, old->egid) || 158 gid_eq(new_id.gid, old->sgid)) 159 return true; 160 } else /* Error, new_type is an invalid type */ 161 return false; 162 163 /* 164 * Transitions to new UIDs require a check against the policy of the old 165 * RUID. 166 */ 167 permitted = 168 setid_policy_lookup((kid_t){.uid = old->uid}, new_id, new_type) != SIDPOL_CONSTRAINED; 169 170 if (!permitted) { 171 if (new_type == UID) { 172 pr_warn("UID transition ((%d,%d,%d) -> %d) blocked\n", 173 __kuid_val(old->uid), __kuid_val(old->euid), 174 __kuid_val(old->suid), __kuid_val(new_id.uid)); 175 } else if (new_type == GID) { 176 pr_warn("GID transition ((%d,%d,%d) -> %d) blocked\n", 177 __kgid_val(old->gid), __kgid_val(old->egid), 178 __kgid_val(old->sgid), __kgid_val(new_id.gid)); 179 } else /* Error, new_type is an invalid type */ 180 return false; 181 } 182 return permitted; 183 } 184 185 /* 186 * Check whether there is either an exception for user under old cred struct to 187 * set*uid to user under new cred struct, or the UID transition is allowed (by 188 * Linux set*uid rules) even without CAP_SETUID. 189 */ 190 static int safesetid_task_fix_setuid(struct cred *new, 191 const struct cred *old, 192 int flags) 193 { 194 195 /* Do nothing if there are no setuid restrictions for our old RUID. */ 196 if (setid_policy_lookup((kid_t){.uid = old->uid}, INVALID_ID, UID) == SIDPOL_DEFAULT) 197 return 0; 198 199 if (id_permitted_for_cred(old, (kid_t){.uid = new->uid}, UID) && 200 id_permitted_for_cred(old, (kid_t){.uid = new->euid}, UID) && 201 id_permitted_for_cred(old, (kid_t){.uid = new->suid}, UID) && 202 id_permitted_for_cred(old, (kid_t){.uid = new->fsuid}, UID)) 203 return 0; 204 205 /* 206 * Kill this process to avoid potential security vulnerabilities 207 * that could arise from a missing allowlist entry preventing a 208 * privileged process from dropping to a lesser-privileged one. 209 */ 210 force_sig(SIGKILL); 211 return -EACCES; 212 } 213 214 static int safesetid_task_fix_setgid(struct cred *new, 215 const struct cred *old, 216 int flags) 217 { 218 219 /* Do nothing if there are no setgid restrictions for our old RGID. */ 220 if (setid_policy_lookup((kid_t){.gid = old->gid}, INVALID_ID, GID) == SIDPOL_DEFAULT) 221 return 0; 222 223 if (id_permitted_for_cred(old, (kid_t){.gid = new->gid}, GID) && 224 id_permitted_for_cred(old, (kid_t){.gid = new->egid}, GID) && 225 id_permitted_for_cred(old, (kid_t){.gid = new->sgid}, GID) && 226 id_permitted_for_cred(old, (kid_t){.gid = new->fsgid}, GID)) 227 return 0; 228 229 /* 230 * Kill this process to avoid potential security vulnerabilities 231 * that could arise from a missing allowlist entry preventing a 232 * privileged process from dropping to a lesser-privileged one. 233 */ 234 force_sig(SIGKILL); 235 return -EACCES; 236 } 237 238 static int safesetid_task_fix_setgroups(struct cred *new, const struct cred *old) 239 { 240 int i; 241 242 /* Do nothing if there are no setgid restrictions for our old RGID. */ 243 if (setid_policy_lookup((kid_t){.gid = old->gid}, INVALID_ID, GID) == SIDPOL_DEFAULT) 244 return 0; 245 246 get_group_info(new->group_info); 247 for (i = 0; i < new->group_info->ngroups; i++) { 248 if (!id_permitted_for_cred(old, (kid_t){.gid = new->group_info->gid[i]}, GID)) { 249 put_group_info(new->group_info); 250 /* 251 * Kill this process to avoid potential security vulnerabilities 252 * that could arise from a missing allowlist entry preventing a 253 * privileged process from dropping to a lesser-privileged one. 254 */ 255 force_sig(SIGKILL); 256 return -EACCES; 257 } 258 } 259 260 put_group_info(new->group_info); 261 return 0; 262 } 263 264 static struct security_hook_list safesetid_security_hooks[] = { 265 LSM_HOOK_INIT(task_fix_setuid, safesetid_task_fix_setuid), 266 LSM_HOOK_INIT(task_fix_setgid, safesetid_task_fix_setgid), 267 LSM_HOOK_INIT(task_fix_setgroups, safesetid_task_fix_setgroups), 268 LSM_HOOK_INIT(capable, safesetid_security_capable) 269 }; 270 271 static int __init safesetid_security_init(void) 272 { 273 security_add_hooks(safesetid_security_hooks, 274 ARRAY_SIZE(safesetid_security_hooks), "safesetid"); 275 276 /* Report that SafeSetID successfully initialized */ 277 safesetid_initialized = 1; 278 279 return 0; 280 } 281 282 DEFINE_LSM(safesetid_security_init) = { 283 .init = safesetid_security_init, 284 .name = "safesetid", 285 }; 286